Oxalic acid is a dicarboxylic acid of the formula HO2C—CO2H. Oxalic acid exists primarily as oxalate in biological organisms, which is the salt form of oxalic acid. Oxalate is found in foods, such as, e.g., spinach, rhubarb, strawberries, cranberries, nuts, cocoa, chocolate, peanut butter, sorghum, and tea. Oxalate is also a metabolic end product in humans and other mammals. It is excreted by the kidneys into the urine. When combined with calcium, oxalic acid produces an insoluble product, calcium oxalate, which is the most prevalent chemical compound found in kidney stones.
Because mammals do not synthesize enzymes that degrade oxalate, oxalate levels in an individual are normally held in check by excretion and low absorption of dietary oxalate. Elevated concentrations of oxalate are associated with a variety of pathologies, such as primary hyperoxaluria, enteric hyperoxaluria, and idiopathic hyperoxaluria. Leumann et al., Nephrol. Dial. Transplant. 14:2556-2558 (1999) and Earnest, Adv. Internal Medicine 24:407-427 (1979). Increased oxalate can be caused by consuming too much oxalate from foods, by hyperabsorption of oxalate from the intestinal tract, and by abnormalities of oxalate production. Hyperabsorption of oxalate in the colon and small intestine can be associated with intestinal diseases, including hyperabsorption caused by diseases of bile acid and fat malabsorption; ileal resection; and, for example, by steatorrhea due to celiac disease, exocrine pancreatic insufficiency, intestinal disease, and liver disease.
Hyperoxaluria, or increased urinary oxalate excretion, is associated with a number of health problems related to the deposit of calcium oxalate in the kidney tissue (nephrocalcinosis) or urinary tract (e.g., kidney stones, urolithiasis, and nephrolithiasis). Calcium oxalate may also be deposited in, e.g., the eyes, blood vessels, joints, bones, muscles, heart and other major organs, causing damage to the same. See, e.g., Leumann et al., J. Am. Soc. Nephrol. 12:1986 1993 (2001) and Monico et al., Kidney International 62:392 400 (2002). The effects of increased oxalate levels can appear in a variety of tissues. For example, deposits in small blood vessels cause painful skin ulcers that do not heal, deposits in bone marrow cause anemia, deposits in bone tissue cause fractures or affect growth in children, and calcium oxalate deposits in the heart cause abnormalities of heart rhythm or poor heart function.
Existing methods to treat elevated oxalate levels are not always effective and intensive dialysis and organ transplantation may be required in many patients with primary hyperoxaluria. Existing therapies for various hyperoxalurias include high-dose pyridoxine, orthophosphate, magnesium, iron, aluminum, potassium citrate, cholestyramine, and glycosaminoglycan treatment, as well as regimes for adjusting diet and fluid intake, for dialysis, and for surgical intervention, such as renal and liver transplantation. These therapies (e.g., low-oxalate or low-fat diet, pyridoxine, adequate calcium, and increased fluids), are only partially effective and they may have undesirable adverse side effects, such as the gastrointestinal effects of orthophosphate, magnesium, or cholestyramine supplementation and the risks of dialysis and surgery. Accordingly, methods that safely remove oxalate from the body are needed. Moreover, methods that degrade oxalate to reduce oxalate levels in a biological sample are advantageous over a therapy, for example, that solely blocks absorption or increased clearance of oxalate.